File with LikeRec Data-Structure Template -vs- ExtName Data-Structure Template

I have a question about the differences/advantages/disadvantages between
these two methods of utilizing a data-structure template containing field
definitions for a physical file.

_______________

Method 1
-Define the physical file as a template
-Define a template data-structure as LikeRec the defined
file_name.format_name
-Refer to the data-structure when defining other fields

example:
dcl-f CUSTFILE keyed template qualified;
dcl-ds cust_t likeRec(CUSTFILE.CUSTFILER) template;
dcl-s custName like(cust_t.name);

_______________

Method 2
-Define a template data-structure as ExtName the physical file
-Refer to the data-structure when defining other fields

example:
dcl-ds cust_t extName('CUSTFILE') qualified template end-ds;
dcl-s custName like(cust_t.name);

_______________

The few questions I have:
1. Is there a benefit to method 1, considering the requirement to define
the file in the F specs?
2. I would assume in method 1 that since the file is defined as a
template, if a new field were to be added to the file, the program would
not need to be re-compiled unless it needed to refer to that new field.
And I would make the same assumption for method 2, as the file isn't
defined at all. Am I correct in my reasoning?
3. If there is no advantage to method 1, does anybody have a good reason
to use template defined f-specs?
4. Why is the end-ds required at the end of an ExtName data-structure, but
not LikeRec?

Thanks in advance for any insight!